Yuga is the first major boss you'll face, encountered at the end of the Eastern Palace. He has two distinct phases, both requiring you to utilize your sword and dodge his attacks. Ensure you have the Bow rented, as it's helpful for the second phase.
Phase 1: Painting Attacks
Yuga will transform into a painting and move along the walls, occasionally emerging to summon Popos or fire energy blasts. Your primary goal is to wait for him to fully emerge from the wall. When he does, quickly rush in and strike him with your sword. He will then retreat back into the wall. Dodge his energy blasts by moving perpendicular to their trajectory. The Popos he summons are easily dispatched with a few sword swings.
Phase 2: Armos Knight Paintings
After taking enough damage, Yuga will begin to create paintings of Armos Knights that come to life. These Armos Knights are invulnerable to your sword from the front. You need to use your Bow to shoot their glowing eyes. Once an Armos Knight is defeated, Yuga will emerge from the wall again. This is your window to attack him with your sword. Repeat this process, prioritizing the Armos Knights to clear the way for attacking Yuga. Stay mobile and keep your distance when he's on the wall. Defeating him rescues Seres and grants you the Pendant of Power.